Role of Bottom-up and Top-Down approaches in Nano technology:                                                                                                                                                                               BACK



Figure: Schematic representation of the building up of Nanostructures.


There are two approaches for synthesis of nano materials and the fabrication of nano structures. Top down approach refers to slicing or successive cutting of a bulk material to get nano sized particle.     Bottom up approach refers to the build up of a material from the bottom: atom by atom, molecule by molecule or cluster by cluster.

            Both approaches play very important role in modern industry and most likely in nano technology as well. There are advantages and disadvantages in both approaches.

            Attrition or Milling is a typical top down method in making nano particles, where as the colloidal dispersion is a good example of bottom up approach in the synthesis of nano particles.

            The biggest problem with top down approach is the imperfection of surface structure and significant crystallographic damage to the processed patterns. These imperfections which in turn leads to extra challenges in the device design and fabrication. But this approach leads to the bulk production of nano material. Regardless of the defects produced by top down approach, they will continue to play an important role in the synthesis of nano structures.

            Though the bottom up approach oftenly referred in nanotechnology, it is not a newer concept. All the living beings in nature observe growth by this approach only and also it has been in industrial use for over a century. Examples include the production of salt and nitrate in chemical industry.

            Although the bottom up approach is nothing new, it plays an important role in the fabrication and processing of nano structures. There are several reasons for this and explained as below.

            When structures fall into a nanometer scale, there is a little chance for top down approach. All the tools we have possessed are too big to deal with such tiny subjects. Bottom up approach also promises a better chance to obtain nano structures with less defects, more homogeneous chemical composition.

            On the contrary, top down approach most likely introduces internal stress, in addition to surface defects and contaminations.